Increasing capacity and higher functionality of wireless apparatuses in general are accompanied by further demands for lower power consumption, downsizing, lower costs, and higher stability. Therefore, the wireless apparatus is generally provided with a distortion compensating circuit in a power amplifier.
To obtain the effect of the distortion compensating circuit, it is necessary to enhance the stability of a distortion component and an electric power component. Therefore, a high performance feedback circuit is used in the distortion compensating circuit (see, e.g., Japanese Patent Application Laid-open No. 2001-57522).
A conventional distortion compensating circuit provided with a high performance feedback circuit is described in detail below. FIG. 5 depicts a distortion compensating circuit according to a conventional technique. As depicted in FIG. 5, the distortion compensating circuit includes a signal processing/control unit, a TRF unit (transmission high frequency circuit), a PA unit (high-power amplifying circuit), and a VSWR unit (Voltage Standing Wave Ratio circuit).
The signal processing/control unit produces and outputs a transmission signal (modulated wave). The TRF unit has an FW-MOD (Feedforward-Modulation) that converts the transmission signal output from the signal processing/control unit into an RF signal (high frequency signal) and an FB-CONV (FeedBack-Converter) that is a feedback circuit. The PA unit has a directional coupler (DC) that amplifies the RF signal converted by the TRF unit to a desired value and that separates the RF signal into electric power to be output to an antenna (ANT) not shown and electric power to be feed-back. The VSWR unit has a VSWR (Voltage Standing Wave Ratio) that outputs the electric power amplified by the PA unit to the antenna, and that detects the state of the antenna from the electric power output to the antenna and that outputs the state to the signal processing/control unit.
The distortion compensating circuit having such a configuration outputs the transmission signal from the signal processing/control unit (see (1) of FIG. 5). The distortion compensating circuit separates the output transmission signal into the electric power for the ANT and that for the FB-CONV using the PA unit located at around the final output (see (2) of FIG. 5). The distortion compensating circuit feeds back the signal separated for the FB-CONV to the signal processing/control unit to analyze the electric power and the distortion component of the signal output from the ANT at the location (see (3) of FIG. 5). The signal processing/control unit to which the output signal is fed back analyzes the signal. When the signal component analyzed includes much distortion, the signal processing/control unit determines that the signal component radiated from the ANT includes much distortion. The signal processing/control unit also determines whether the electric power of the signal is high or low. Based on these determination results, the signal processing/control unit executes distortion compensation to correct the distortion component (distortion information) and the amplitude component (electric power information) to the signal to be output to the FW-MOD of the TRF unit, and the signal processing/control unit outputs the compensated signal. The distortion compensation is an ordinary compensation executed by calculating a compensation coefficient using the distortion component and the amplitude component, and using the calculated compensation coefficient. Therefore, the detailed description for the compensation is not provided.
Referring to FIG. 6, the process flow of a distortion compensation according to the above conventional technique is described. FIG. 6 is a flowchart of the distortion compensating process according to the conventional technique. As illustrated in FIG. 6, in the distortion compensating circuit according to the conventional technique: the signal processing/control unit produces a modulated wave (IQ signal) using a compensation coefficient at the current stage and outputs the modulated wave to the TRF unit (Step S201); the FW-MOD of the TRF unit converts the output modulated wave into an RF signal (Step S202) and, thereafter, a PA unit amplifies the RF signal to a desired value (ANT output value) (Step S203).
The distortion compensating circuit radiates the signal amplified by the PA unit from the ANT through the VSWR unit (Steps S204 and S205) and outputs a portion of the signal to an FB-CONV that is a feedback circuit (Step S206). The distortion compensating circuit that feeds back the portion of the output signal to the antenna produces an IF signal (Intermediate Frequency) by executing frequency conversion and power amplification to the signal fed back (Step S207). The distortion compensating circuit executes temperature compensation in the feedback circuit to the produced IF signal to reduce the influence received from the external environmental variation (for example, temperature) (Step S208) and the IF signal is separated into its modulation component and its signal power through an A/D conversion (Analog/Digital conversion) (Step S209).
The distortion compensating circuit that obtains the modulation component and the signal power in this manner measures and compares a distortion component from the obtained modulation component (Step S210), also measures and compares an amplitude component from the obtained signal power (Step S211), and calculates a compensation coefficient from both of the distortion component and the amplitude component (Step S212). Thereafter, the distortion compensating circuit repeats the process of Step S201 and its succeeding steps using the newly calculated compensation coefficient.
However, the above conventional technique has difficulties in feeding back stable information especially on electric power information and, therefore, proper distortion compensation is not executable. Further, the conventional technique has a problem in that, even when stable information on the electric power can be fed back, it takes long to adjust the feedback circuit and, in addition, the feedback circuit becomes complicated and expensive.
More specifically, when distortion compensation is executed, the signal returned from the feedback circuit and the signal information actually radiated from the ANT need to be matched with each other and, to do so, especially the electric power information is required to be fed back at high stability. However, many elements such as a DC-VATT (variable attenuator), DC-MIX (mixer), DC-AMP (amplifier), DC-AEQL (Amplitude Equalizer), DC-DEQL (Delay Equalizer), and DC-AMP are used in the feedback circuit (see FIG. 5), and each of the elements has its own characteristic against the environmental temperature. Particularly, an active element (such as an AMP) receives a stronger influence from the environmental temperature compared to a passive element and its variation rate of the gain per one degree Centigrade [° C.] is generally 0.015 decibel [dB] (for a GaAs FET). Therefore, the distortion compensating circuit according to the conventional technique executes amplitude (electric power) compensation using a TH (gain-temperature compensating circuit). However, there is a limit in properly compensating the amplitude (electric power) for each of the above many elements based on the environmental temperature.
It is conceivable to compensate the amplitude (electric power) for each of the above many elements by using a feedback circuit that includes many THs. However, in this case, the feedback circuit becomes complicated and, because of the need to increase the elements, the circuit becomes expensive. In addition, each TH must be adjusted to compensate the amplitude (electric power) and, the adjustments take tremendous time.